U.S. Pat. No. 3,656,828 relates to a device for producing a spatial reference plane. This known device comprises a laser which emits a light beam towards a rotating scanning head. From this head two laser beams are transmitted, one of which is directed at a small angle above the wanted reference system and the other at a small angle below the same reference plane. As a result the two beams on rotation form two conical surfaces. However, in these known devices the accuracy of the determination of the reference plane will decrease with increasing distance to the device, since the two beams form a certain angle with the plane to be indicated and consequently diverge.